Light-sensitive silver halide print-out emulsions



United States Patent Int. Cl. G03c 1/28 U.S. Cl. 96-108 4 Claims ABSTRACT OF THE DISCLOSURE Preparation of improved print-out emulsion by sequential addition to light-sensitive gelatin-silver bromide emulsion, at a pH up to 5, of K1, Pb(NO and KBr.

The present invention relates to light-sensitive silver halide print-out emulsions. It relates particularly to emulsions suitable for use in oscillograph recording instruments and the like, for example, those instruments wherein signals from an external source are transformed as by mirror galvanometers or equivalents into oscillating beams of light. These beams of oscillating light are focused through an optical system onto the recording paper.

The light source commonly used in oscillograph recording instruments is a high pressure mercury vapor lamp and the light impinging on the sensitive paper at the point of recording is normally quite intense. Hence, the emulsion must be sensitive to this type of illumination. The invention contemplates also a supplementary treatment or operation on the exposed record, wherein the barely visible latent image of the graph is latensified or increased in density by overall exposure to ambient light, preferably fluorescent or ultraviolet light, to yield a strong and fully visible print-out image. This latensified image is usually and preferably in the 0.3 to 0.6 reflectance density range.

The print-out emulsions of the present invention exhibit a pronounced sensitivity to exposures of high intensity and short duration, and a very low sensitivity to exposures of low intensity and long duration. As a matter of fact, these low intensity exposures of long duration have a desensitizing effect. For this reason, the background fog level remains relatively low during the latensification procedure and also during subsequent examination of the latensified material under ambient light.

Print-out emulsions for recording oscillographs are well known in the art and have been described, for instance in U.S. Patent 3,039,871. The silver halide employed in the above-described print-out emulsions normally have been combined with a mixture of lead iodide and lead bromide in a comparatively slow and time consuming colloid milling operation.

The procedure of the present invention utilizes a much simpler technique and does not necessitate the use of special equipment. Besides, the emulsions obtained have better stability and produce images of greater density.

To produce print-out emulsions of good quality, it appears important to have individual ions of lead, bromide and iodide present, preferably incorporated in the sequence: iodide ions, lead ions and bromide ions. All three must be added to the emulsion in this order to produce a latent image from the high intensity light. This image is capable of rapid latensification by ambient light; at the same time, the background fog density is kept at a satisfactorily low level.

Data were obtained showing that the iodide ion is important for the photolytic (print-out) reaction, and that both the lead and the bromide ions, but especially the lead ions, are essential to minimize the background fog formation. These data are summarized below:

TABLE I Ion Print-out density Background fog density High High Low Low Low Low High High High Medium Low Low High Low The above table indicates in the last line, that the three ions, when combined properly and in the correct sequence, give the desired results of high print-out density and low background fog density.

Many photographic emulsions are commercially available which can be converted or modified to produce satisfactory print-out emulsions for oscillograph record ing and analogous uses. Emulsions prepared according to the present invention, are first ripened in the presence of potassium iodide, then treated with lead nitrate and finally treated with potassium bromide to form the desired lead bromide (PbBr The small amount of iodide which does not enter the silver halide lattice as AgI during the ripening procedure is probably converted to Pbl Instead of selecting a conventional silver bromide emulsion for the primary emulsion, which is used as the starting material, it has been found advantageous to formulate special emulsions by a special technique. Such silver halide emulsions are produced when a silver chloride or a silver chloro-bromide emulsion is digested with a sufficient amount of potassium bromide to convert this silver chloride containing emulsion into one which consists entirely of silver bromide. Iodide enters the silver halide crystal lattice when an alkali iodide is subsequently added during the ripening step described above. The sequential silver chloride/ silver bromide/ silver iodide procedure results in the formation of silver halide (silver bromoiodide) crystals which have a pronounced sensitivity to light of high intensity and short duration. Thus, emulsions containing such silver bromoiodide crystals are very sensitive to the high intensity light beams of oscillograph traces, xenon flash tubes, or high pressure mercury vapor arc lights. The residual or background sensitivity of these silver halide crystals to illumination of low intensity, such as ambient light, can be reduced by treating the emulsions with the water-soluble salt of a divalent lead ion, Pb++ followed by a treatment with a water-soluble alkali metal bromide as described above. The overall result, then, is the formation of a print-out emulsion system which is eminently suitable for the recording of oscillograph traces or high intensity beams of light from the above mentioned sources, including high intensity incandescent light sources which provide electromagnetic energy in the visible and ultraviolet range.

The improved oscillograph recording print-out emulsions when coated on a suitable support, such as paper, have a higher fresh net density, a faster latensification rate, a greater stability and a better sharpness than the emulsions described by the prior art. Moreover, as indicated above, the time consuming colloid milling operation is no longer needed for the blending of the basic ingredients.

This product also has the advantage that the amounts of silver halide used are rather moderate. A considerable saving in silver can be achieved because the emulsion can be coated on record paper to a (metallic) silver weight of about 2.0 to 4.0 grams per square meter to produce an entirely satisfactory record. The record paper prepared with the emulsion of the present invention also can readily be permanized by processing in special developers containing a developing agent and a silver halide solvent to render the image and the background completely insensitive to further illumination.

The type of gelatin, the silver to gel ratio, the silver chlombromide-iodide ratio, the concentration of reagents, the pH and the bromide content of the emulsion, the addition times and the temperatures all may be optimized to yield the highest trace print-out density, the fastest latensification rate and the lowest background fog level consistently obtainable. It is possible, however, to deviate considerably from the detailed specifications given below without causing serious variance in the qualities and characteristics of the emulsion.

The preferred and limiting values of the various factors, materials and operating conditions are listed below as follows:

(1) Inert gelatin is preferred to active or sensitizing gelatin in order to reduce the surface sensitivity during the ripening.

(2) An ammonia type emulsion is preferred to the boiled type because this permits the attainment of an average silver halide crystal size of approximately 0.5 to 5.0 microns. This is considered near optimum for this type of record emulsion.

(3) The making temperature can be varied considerably, preferably between about 40" C. and 60 C.

(4) The silver chloride-bromide ratio of the starting emulsion can be varied all the way from 100 percent silver chloride to 100 percent silver bromide. The preferred composition, however, is about 60 to 80 mole percent silver chloride and 20 to 40 mole percent silver bromide.

(5) The starting emulsion is preferably converted to one which contains essentially 100 percent silver bromide. It is possible, however, to operate with only partial conversion or even without conversion and still have a workable composition.

(6) The pH of the ripened emulsion is lowered and adjusted to a value below 7.0 and preferably to a. value between 1.8 and 5.0 to reduce background fog level. Any suitable acid, such as sulfa-mic acid, citric acid or tartaric acid may be used for this purpose. pH values above 7.0 are undesirable since they tend to increase fog density.

(7) The ripening temperature can be varied from about 40 C. to 70 C. and the total ripening time may be varied between about minutes and 2 hours.

(8) As previously indicated, the emulsion should be ripened with the addition of potassium iodide, lead nitrate and potassium bromide in the sequence just stated. If the sequence is changed, the emulsion will exhibit a lower net trace density, meaning, that the density difierence between the trace image and the background will be reduced. Hence, the proper sequence of addition is important.

(9) The quantity of potassium iodide which is added as a ripening final can be varied all the way from 0.5 to mole percent based on the silver content. An optimum is considered about 2.5 to 7.5 mole percent.

(10) The quantity of lead bromide which is incorporated in the emulsion by addition of lead nitrate and potassium bromide can also be varied from 0.5 to 20 mole percent. At its lower value, the lead bromide gives the best trace density, but also yields the highest background fog density. Higher values give lower trace densities, but also lower background fog level. About 5 to 12.0 mole percent of lead bromide based on the total silver content of the emulsion appears to be optimum.

(11) The finished emulsion is coated on paper to a coating weight of about 2.0 to 4.0 grams of silver per square meter. When exposed in the oscillogra-ph or to other high intensity light sources and latensified by exposure with either fluorescent or ultraviolet light, this coated paper is capable of yielding a net trace reflectance density of approximately 0.30 to 0.60 unit.

(12) The stability of the coated emulsion can be improved and the fresh net trace density can be increased by an additional 0.1 to 0.2 unit by applying an additional surface coating, The emulsion layer is preferably overcoated with a gelatin solution which contains the alkali metal salt of a weak acid (20.0 grams to 40.0 grams of sodium formate, sodium acetate or borax per liter of 2.0 percent surface gel). These salts, in combination with the previously described acids, form a buffering system. The preferred alkaline ingredient is sodium formate.

The above print-out emulsions can be optically sensitized with orthochromatic sensitizing dyes of the type described in US. Patents 2,159,037, 2,173,486 and 2,233,511.

These emulsions are then suitable for use in the above described oscillographs which employ high intensity incandescent light sources.

Instead of using gelatin as the basis of the emulsion, other colloidal carrier materials, such as polyvinyl alcohol, hydroxyethyl cellulose or polyvinyl pyrrolidone can be used. The invention is further illustrated by the following examples:

EXAMPLE I In a preferred form, the procedure of my invention starts with a combination of from 60 to molar equivalents of potassium bromide and 20 to 40 molar equivalents of sodium chloride in a gelatin solution. The corresponding silver halide elulsion is produced with 100 molar equivalents of ammoniacal silver nitrate solution. The resulting silver halide emulsion contains 60 to 80 mole percent of the chloride and 20 to 40 mole percent of the bromide. Then, additional potassium bromide is added to convert the silver bromochloride to the pure silver bromide. This preliminary silver bromide gelatin emulsion is then precipitated, for instance with 50 percent ammonium sulfate solution, washed and reconstituted with a gelatin solution. Five mole percent of potassium iodide is now added, which results in a silver halide composition of about mole percent bromide and 5 mole percent iodide.

By adding in succession to the above silver bromoiodide emulsion, sulfamic acid, 5.0 mole percent of lead nitrate and 10.0 mole percent of potassium bromide, the resulting composition consists of a silver bromoiodide emulsion with about 5.0 mole percent of lead bromide based on the total silver content in the emulsion.

In more detail, the operation is carried out as follows:

EXAMPLES II TO IV Three separate batches of an ammoniacal silver nitrate solution were prepared as follows:

Solution 1 Silver nitrate (4.7 moles) grams 800.0 Ammonium hydroxide (28%) ml 800.0 Water liters 2.5

Solution 2 Each of these three solutions was added with stirring to one of the following solutions:

Solution 2a Sodium chloride (4.98 moles) grams 292.0 Gelatin do 50.0 Water liters 2.5

Solution 3a Sodium chloride (2.63 moles) grams 154.0 Potassium bromide (2.35 moles) do 280.0 Gelatin do 50.0 Water liters 2.5

Solution 4a Potassium bromide (4.98 moles) grams 594.0 Gelatin do.. 500 Water liters 2.5

In order to convert these emulsions into silver bromide emulsions, compositions 2b and 3b were added at a temperature of 45 C. with stirring to the silver halide emulsion obtained from the reaction of Solutions 2a and 3a with Solution 1.

The emulsion derived from compositions l and 4a was a pure silver bromide emulsion and did not require any addition.

Compositions 2b and 3b contained the following ingredient:

Solution 2b Potassium bromide (4.98 moles) grams 594.0 Water liters 1.0

Solution 3b Potassium bromide (2.63 moles) grams 310.0 Water liters 1.0

These emulsions were then digested at 45 C. and thereafter precipitated with a 50 percent ammonium sulfate solution. The precipitates were then washed four times With Water by decantation and reconstituted with 460 grams of gelatin in 1.5 liters of Water. Sufiicient water was then added to make 4 kilograms of emulsion each.

These emulsions were then treated in sequence and with vigorous stirring at a temperature of 50 C. with 400 milliliters of a percent sulfamic acid solution and 400 milliliters of a 10 percent potassium iodide solution. At this point the emulsions were digested for minutes. 800 milliliters of a 10 percent lead nitrate solution and 600 milliliters of a 10 percent solution potassium bromide solution were then added and the mixture digested for 15 minutes. The resultant emulsions were coated on a baryta treated paper stock and dried. Exposure to the light beam of an oscillograph produced high density black trace image on a faint yellow background after latensification by exposure to ambient room daylight, to ultraviolet light or fluorescent light.

Modifications of this invention will thus occur to persons skilled in the art. I, therefore, do not intend to be limited in the patent granted except as necessitated by the appended claims.

What is claimed is:

1. The process of preparing a print-out emulsion suitable for use in oscillograph recordings, which comprises starting with a mixture of inert gelatin and a silver bromide, adjusting the pH of the mixture to not higher than 5.0, adding in sequence first 0.5 to 20 mole percent of potassium iodide based on total silver content, and then from 0.5 to 20 mole percent of lead nitrate and finally an amount of potassium bromide at least essentially equivalent to said lead nitrate and ripening said emulsion.

2. Process according to claim 1 wherein the proportions of potassium iodide are about 5 mole percent and the proportions of lead nitrate about 5 mole percent, based on the total amount of silver.

3. A process according to claim 1 wherein said silver bromide is prepared by treating a silver chloride emulsion With an alkali metal bromide.

4. A process according to claim 1 wherein said silver bromide is prepared by treating a silver chloro-bromide emulsion with analkali metal bromide.

3/1964 Byme 96108 5/1966 Hunt 96-108 NORMAN G. TORCHIN, Primary Examiner. R. E. MARTIN, Assistant Examiner.

US. Cl. X.R. 9694 

